Statins are currently the most therapeutically effective drugs available for reducing low-density lipoprotein (LDL) particle concentration in the blood stream of patients at risk for cardiovascular disease. Thus, statins are used in the treatment of hypercholesterolemia, hyperlipoproteinemia, and atherosclerosis. A high level of LDL in the bloodstream has been linked to the formation of coronary lesions that obstruct the flow of blood and can rupture and promote thrombosis. Goodman and Gilman, The Pharmacological Basis of Therapeutics, page 879 (9th Ed. 1996).
Statins inhibit cholesterol biosynthesis in humans by competitively inhibiting the 3-hydroxy-3-methyl-glutaryl-coenzyme A (“HMG-CoA”) reductase enzyme. HMG-CoA reductase catalyzes the conversion of HMG to mevalonate, which is the rate-determining step in the biosynthesis of cholesterol. Decreased production of cholesterol causes an increase in the number of LDL receptors and corresponding reduction in the concentration of LDL particles in the bloodstream. Reduction in the LDL level in the bloodstream reduces the risk of coronary artery disease. J.A.M.A. 1984, 251, 351-74.
Currently available statins include lovastatin, simvastatin, pravastatin, fluvastatin, cerivastatin and atorvastatin. Lovastatin (disclosed in U.S. Pat. No. 4,231,938) and simvastatin (disclosed in U.S. Pat. No. 4,444,784) are administered in the lactone form. After absorption, the lactone ring is opened in the liver by chemical or enzymatic hydrolysis, and the active hydroxy acid is generated.
Pravastatin (disclosed in U.S. Pat. No. 4,346,227) is administered as the sodium salt. Fluvastatin (disclosed in U.S. Pat. No. 4,739,073) and cerivastatin (disclosed in U.S. Pat. Nos. 5,006,530 and 5,177,080), also administered as the sodium salt, are entirely synthetic compounds that are in part structurally distinct from the fungal derivatives of this class that contain a hexahydronaphthalene ring. Atorvastatin and two new “superstatins,” rosuvastatin and pitavastatin, are administered as calcium salts.
Rosuvastatin calcium (monocalcium bis(+)7-[4-(4-fluorophenyl)-6-isopropyl-2-(N-methyl-N-methylsulfonylaminopyrimidin)-5-yl]-(3R,5S)-dihydroxy-(E)-6-heptanoate) is an HMG-CoA reductase inhibitor, developed by Shionogi for the once daily oral treatment of hyperlipidaemia (Ann Rep, Shionogi, 1996; Direct communications, Shionogi, 8 Feb. 1999 & 25 Feb. 2000). Rosuvastatin calcium has the following chemical formula:

Rosuvastatin calcium is marketed under the name CRESTOR® for treatment of a mammal such as a human. According to the maker of CRESTOR®, it is administered in a daily dose of from about 5 mg to about 40 mg. For patients requiring less aggressive LDL-C reductions or who have pre-disposing factors for myopathy, the 5 mg dose is recommended, while 10 mg dose is recommended for the average patient, 20 mg dose for patients with marked hyper-cholesterolemia and aggressive lipid targets (>190 mg/dL), and the 40 mg dose for patients who have not been responsive to lower doses.
U.S. Pat. No. 5,260,440 discloses and claims rosuvastatin, its calcium salt (2:1), and its lactone form. The process of the '440 patent prepares rosuvastatin by reacting 4-(4-fluorophenyl)-6-isopropyl-2-(N-methyl-N-methylsulfonylamino)-5-pyrimidinecarbaldehyde with methyl (3R)-3-(tert-butyldimethylsilyloxy)-5-oxo-6-triphenylphosphoranylidene hexanoate in acetonitrile under reflux. The silyl group is then cleaved with hydrogen fluoride, followed by reduction with sodium borohydride (NaBH4) and diethylmethoxyborane in tetrahydrofuran (THF) to obtain a methyl ester of rosuvastatin.
The ester is then hydrolyzed with sodium hydroxide (NaOH) in ethanol at room temperature, followed by removal of ethanol and addition of ether, to obtain the sodium salt of rosuvastatin. The sodium salt is then converted to the calcium salt. The sodium salt is dissolved in water and maintained under a nitrogen atmosphere. Calcium chloride is then added to the solution, resulting in precipitation of rosuvastatin calcium (2:1). The process for preparation of the intermediates disclosed in the '440 patent is incorporated herein by reference.
The product mixture of a reaction rarely is a single compound pure enough to comply with pharmaceutical standards. Side products and byproducts of the reaction and adjunct reagents used in the reaction will, in most cases, be present. At certain stages during processing of the rosuvastatin contained in the product mixture into an active pharmaceutical ingredient (“API”), the rosuvastatin must be analyzed for purity, typically by HPLC or GC analysis, to determine if it is suitable for continued processing or ultimately for use in a pharmaceutical product. The rosuvastatin does not need to be absolutely pure. Absolute purity is a theoretical ideal that is unattainable. Rather, there are purity standards intended to ensure that an API is not made less safe for clinical use because of the presence of impurities. In the United States, the Food and Drug Administration guidelines recommend that applicants limit some impurities to below 0.1%.
Generally, side products, byproducts and adjunct reagents (collectively “impurities”) are identified spectroscopically and by other physical methods and then the impurities are associated with a peak position in a chromatogram (or a spot on a TLC plate). (Strobel p. 953) (Strobel, H. A.; Heineman, W. R., Chemical Instrumentation: A Systematic Approach, 3rd dd. (Wiley & Sons: New York 1989)). Thereafter, the impurity can be identified by its position in the chromatogram, which is conventionally measured in minutes between injection of the sample on the column and elution of the particular component through the detector, known as the “retention time.” This time period varies daily based upon the condition of the instrumentation and many other factors. To mitigate the effect that such variations have upon accurate identification of an impurity, practitioners use “relative retention time” (“RRT”) to identify impurities. (Strobel p. 922). The RRT of an impurity is its retention time divided by the retention time of some reference marker. In theory, rosuvastatin itself could be used as the reference marker, but as a practical matter it is present in such overwhelming proportion in the mixture that it tends to saturate the column, leading to irreproducible retention times, i.e., the maximum of the peak corresponding to rosuvastatin tends to wander (Strobel FIG. 24.8(b) p. 879, contains an illustration of the sort of asymmetric peak that is observed when a column is overloaded). Thus, it is sometimes desirable to select an alternative compound that is added to, or is present in, the mixture in an amount significant enough to be detectable and sufficiently low as not to saturate the column and to use that compound as the reference marker.
A compound in a relatively pure state can be used as a “reference standard” (a “reference marker” is similar to a reference standard but it is used for qualitative analysis) to quantify the amount of the compound in an unknown mixture. When the compound is used as an “external standard,” a solution of a known concentration of the compound is analyzed by the same technique as the unknown mixture. (Strobel p. 924, Snyder p. 549) (Snyder, L. R.; Kirkland, J. J. Introduction to Modern Liquid Chromatography, 2nd ed. (John Wiley & Sons: New York 1979)). The amount of the compound in the mixture can be determined by comparing the magnitude of the detector response. See also U.S. Pat. No. 6,333,198, incorporated herein by reference.
The reference standard compound also can be used to quantify the amount of another compound in the mixture if the “response factor,” which compensates for differences in the sensitivity of the detector to the two compounds, has been predetermined. (Strobel p. 894). For this purpose, the reference standard compound may be added directly to the mixture, in which case it is called an “internal standard.” (Strobel p. 925, Snyder p. 552).
The reference standard compound can even be used as an internal standard when the unknown mixture contains some of the reference standard compound by using a technique called “standard addition,” wherein at least two samples are prepared by adding known and differing amounts of the internal standard. (Strobel pp. 391-393, Snyder pp. 571, 572). The proportion of detector response due to the reference standard compound that is originally in the mixture can be determined by extrapolation of a plot of detector response versus the amount of the reference standard compound that was added to each of the samples to zero. (e.g. Strobel, FIG. 11.4 p. 392).
The present invention provides compounds that can be used as a reference standard and reference marker for quantification and identification of rosuvastatin and impurities present in batches of rosuvastatin.
A step in the synthesis of statins is reduction of a ketoester to yield the statin. For example, with fluvastatin, in U.S. Pat. No. 5,354,772, a ketoester of fluvastatin is reduced with EtB3/NaBH4 to obtain a diol ester. In another patent, U.S. Pat. No. 5,189,164 (EP 0 363 934), a ketoester of fluvastatin is reduced with diethylmethoxyborane to provide fluvastatin. Both these US patents relate to a process of purifying the FLV-diol ester by chromatography only. In U.S. Pat. No. 5,260,440, relating to rosuvastatin and in the U.S. Pat. No. 5,856,336, relating to pitavastatin, the statin-diol esters are also isolated by chromatography. In example 8 of WO 03/004455, 6-dibenzylcarbamoyl-5-hydroxy-3-oxo-hexanoic acid tert-butyl ester is reduced by hydrogenation at a pressure of 25 bar, followed by drying of ethyl acetate to obtain a residue having a syn to anti ratio of 7.6 to 1.
Reduction of a ketoester is also disclosed in Tetrahedron 49, 1997-2010 (1993). In the paper, reduction of a ketoester, which is not a particular statin, is carried out by EtB3/NaBH4 or RU-binap to provide a diol ester. In another paper, a ketoester, which is also not any particular statin, is reduced by catecholborane in the optional presence of Rh(PPh3)Cl. JOC 55, 5190-5192 (1990).
The choice of reducing agents is an important factor in obtaining a statin from its corresponding ketoester since it influences the ratio of syn to anti obtained. The United States Pharmacopeia (USP) provides standards regarding the ratio of syn to anti that is used in a statin formulation. The USP requirements dictate use of a reducing agent that allows obtaining a high syn to anti ratio.
There is a need in the art for reducing agents which may be employed on an industrial scale on a cost effective basis, and which provide a high ratio of syn to anti and overall yield.
The diol ester obtained after reduction is usually not isolated, and is hydrolyzed to obtain a salt. For example, in U.S. Pat. No. 5,003,080, the intermediate ester isn't isolated at all. In one instance however, in Journal of Labeled Compounds & Radiopharmaceuticals vol. XLI, pages 1-7 (1988), a fluvastatin diol ester is obtained from hexane containing 3% isopropanol by volume. (See also TETRAHEDRON, VOL. 53 (31), 10659-10670, 1997)
We have yet found additional ways to increase the Syn to anti ratio of statins through isolation of the diol ester.